1. Field of the Invention
The invention relates generally to instruments for measuring fluid flow through a pipe or conduit, and more particularly to those meters which utilize ultrasonic signals for measuring flow rate.
2. Description of the Prior Art
A basic ultrasonic flow meter is shown and described in Lee, U.S. Pat. No. 3,935,735, issued Feb. 3, 1976, and assigned to the assignee of the present invention. This meter uses a pair of transducers mounted on opposite sides of a diametrical section of a pipe, with one of the transducers being positioned downstream from the other. This sets up a diagonal ultrasound path from one transducer to the other. Each transducer is both a transmitter and a receiver of an ultrasonic pulse train. In transmission, electrical signals are converted by the transducers to sound waves, while in reception, the transducers convert sound waves to electrical signals. Two pulse trains are transmitted simultaneously at opposite ends of the ultrasound path and are received a short time later at their opposite destinations.
The electrical signals that are used in generating the ultrasonic waves are generated at a relatively higher carrier frequency signal f.sub.C. A relatively lower frequency signal f.sub.D can be extracted from the carrier signal by signal mixing techniques. The ultrasonic pulse train is generated as a number of cycles of the higher frequency signal f.sub.C for at least one period of the desired, lower frequency signal f.sub.D. The lower frequency signal is obtained by mixing the carrier signal of frequency f.sub.C with a comparable high frequency signal of frequency f.sub.C +f.sub.D. When the two higher frequency signals are mixed, they produce an even higher frequency signal f.sub.2C+D representing the sum of the signals that are mixed and a lower frequency signal f.sub.D that represents the difference between the signals that are mixed. The latter is referred to as a signal at the "difference frequency."
The ultrasonic pulse train at the carrier frequency f.sub.C travels across the flow stream at approximately the speed of sound through the particular flow medium. Depending on the flow velocity of the medium in the direction of the pipe axis, the travel time will be shortened slightly for travel in a downstream direction and the travel time will be lengthened slightly for travel in an upstream direction. For pipes of small diameter, the total travel time for the ultrasonic wave is very short. The measurement of small changes in travel time of the ultrasonic wave is a basic technical problem in metering flow rate.
As further explained in the Lee patent, a mathematical relationship has been developed in which the flow rate is expressed as a function of the elapsed travel times for the two ultrasonic pulse trains, and further as a function of the difference between the travel times in the upstream and downstream directions, respectively. The flow rate (V') can be expressed in terms of some constant (k), the difference between the two travel times (T.sub.12 -T.sub.21) and the two travel times (T.sub.12 and T.sub.21) as follows: ##EQU1##
In a pipe with a diameter of one foot, and a medium in which the speed of a sonic wave is 5000 feet/second when the full-scale fluid flow is 2.5 feet/second, the difference between the two travel times T.sub.12 and T.sub.21 is 0.2 microseconds (0.2.times.10.sup.-6 seconds). For an accuracy of 1% of full scale, a difference in travel times as short as 2 nanoseconds (2.times.10.sup.-9 seconds) must be detected.
The Lee patent further discloses that the difference in the two travel times can be measured by detecting the difference in phase of the two lower frequency signals f.sub.D12 and f.sub.D&lt; which can be extracted from the ultrasonic pulse trains. When the oppositely directed pulse trains are received they are conducted through separate receiving channels, where they are electrically mixed with other signals as described above to obtain the sum and difference frequencies. The difference frequency signals are then extracted using low pass filters. The two resulting difference frequency signals are fed to a zero-crossing detector, which produces an output pulse having a pulse width proportional to the phase angle difference between the two zero-crossing points. This signal is then transmitted to a calculating unit which transforms the pulse width to a time difference and performs a calculation according to equation (1) above to obtain a signal proportional to flow rate.
Following the work represented by the Lee patent, there was another device which improved the resolution of the velocity measurement using a multiple-path filter network to analyze the phase shift of the difference frequency signals that were extracted from the ultrasonic pulse trains. In this device each of the two extracted signals was sampled at eight intervals. The samples were taken as analog signals that were stored in banks of switched capacitor circuits. The storage of analog signals was timed with a clock signal from the transmitter.
The availability of microelectronic processors provides an opportunity for greater resolution of phase differences using Fourier mathematical methods. As fast as these circuits are, they are not fast enough to sense either the carrier frequency signals or the difference frequency signals "on the fly" and perform the other calculating and processing tasks assigned to them. There are several issues presented in the handling of the ultrasonic signals on the receiving side. These include: (1) how to capture the reception signal for processing, (2) how to assure synchronization of two reception signals when the circuitry for detecting these signals responds differently in detecting the two signals, (3) how to measure elapsed travel times for the ultrasonic pulse trains and (4) how to implement Fourier analysis of the sampled wave forms.